This invention relates to the forward link of a CDMA cellular telephone network as shown in FIG. 1. Blocks 101, 102 and 103 are data sources representing the information transmitted to each mobile communicating with the base station. Blocks 104, 105 and 106 are the channel coders that add structured redundancy to the information streams. This coding is used for error correction in the mobile. Channel interleaving is added in blocks 107, 108 and 109 to shuffle the order of the encoded symbols. This improves channel code performance in the presence of burst errors. The signals are then each spread with unique spreading codes generated in blocks 110, 111 and 112, added together and spread with a common spreading code generated in block 113. Orthogonal Walsh codes are commonly used in blocks 110, 111 and 112 and a PN sequence-based code is commonly used in block 113.
For conventional base station operation, block 114 performs the modulation and analog signal processing necessary to transmit the forward channel signal using a single antenna. If transmit diversity is being used, block 114 performs the processing necessary to transmit the forward channel signal using two or more antennas.
The composite forward channel signal travels through the radio channel in block 115. The mobile receiver in block 116 extracts the signal through a despreading process and optionally performs some processing on the different multipath replicas of the received signal. This processing can be used to increase the energy of the desired signal or to cancel interference from the other signals transmitted by the base station. Block 117 de-interleaves the samples of the received signal. The channel decoder in block 118 uses the channel coding added to the information stream to perform error correction and produce an estimate of the transmitted information.
On the CDMA forward link, the amplitude of the desired signal and the level of the interference corrupting that signal will change with time. This makes some channel encoded symbols at the mobile receiver output more reliable than others. Convolutional and turbo codes are typically used for error correction on the CDMA forward link [2, 3, 4]. The channel decoder algorithms for these codes are all improved if they are provided with reliability estimates for each received symbol [5, 6].
This reliability information is typically an estimate of the received energy of an encoded symbol divided by an estimate of the variance of the interference plus noise corrupting that symbol. This signal to interference plus noise ratio (SNIR) for received symbol i is denoted γi and must be calculated at the input to channel decoder block 118. This is equivalent to calculating γi at the output of receiver block 116 since the de-interleaver block 117 does not alter γi.